1. Field of the Invention
The present invention relates to a steering apparatus for a vehicle, in which an operation unit that is operated by a driver and a turning actuator for turning steerable wheels are made mechanically connected or and disconnected.
Priority is claimed on Japanese Patent Applications No. 2005-115545, filed Apr. 13, 2005, the contents of which are incorporated herein by reference.
2. Description of the Related Art
A steer-by-wire steering apparatus, in which a steering wheel (an operation unit) that is operated by a driver and steerable wheels are not mechanically connected, typically includes a reaction force actuator that applies an appropriate reaction force to a driver when the driver operates a steering wheel, and a turning actuator for turning steerable wheels in response to the operation of the steering wheel applied by the driver, and these actuators are independently controlled.
Some of the abovementioned steer-by-wire steering apparatuses include a buckup system in order to ensure steering and turning operations even when a failure occurs in the steering system.
As such a buckup system, a device is known in which a rotation shaft of a steering wheel and an input shaft of a turning mechanism are connected via a planetary gear unit, and one of the gears of the planetary gear unit is selectively made unrotatable using a locking element (see, for example, Japanese Unexamined Patent Application, First Publication No. 2005-29016).
In this buckup system, the steering wheel and the input shaft of the turning mechanism are mechanically disconnected in a normal state by allowing all of the gears of the planetary gear unit to freely rotate so that the steering apparatus functions as a steer-by-wire system, whereas in an abnormal state, the steering wheel and the input shaft of the turning mechanism are mechanically connected by not allowing one of the gears of the planetary gear unit to rotate by locking the gear using the locking element, so that the steering apparatus functions as a steering apparatus in which the steerable wheels can be directly operated and turned by the steering wheel (i.e., functions as a non-steer-by-wire system). Moreover, in this buckup system, the locking element is also operated even when a system failure occurs.
However, there may be a case in which an unlocking operation of the locking element cannot be carried out due to a torsional load that is caused by a torsional phase difference between the steering wheel and the steerable wheels due to the driver's effort applied to the steering wheel in one rotational direction during the unlocking operation of the locking element. In particular, such an unlocking operation tends to be blocked by a great torsional force due to the driver's effort applied to the steering wheel and due to a great load applied to the steerable wheel side of the planetary unit because the steerable wheels are fixed at starting-up of the steering apparatus. If a control operation for a steer-by-wire system is applied to the steering apparatus while the unlocking element is not unlocked in such a case, the driver may feel unusual sensations because the operation of the steering mechanism is restricted by the steering wheel.